Production of silicon steel of uniformly low core loss



Patented Sept. 26, 1944 PBODUGIIO'N OF SILICON STEEL OF UNIFORMLY LOW,CORE LOSS Franklin, and John M.

Victor W. Carpenter,

Middletown,

assignors to .The American Jackson, Butler, Pa.,

Mill Company, Middletown,

tion of Ohio Ohio, and Jack E. Lucas.

Rolling Ohio, :1. corpora- No Drawing. Application August 3, 1940,

Serial No. 351,126

9 Claims.

Our invention relates to the'production of silicon steel for electricuses and has for its principal object the attainment of uniformly lowcore loss in low carbon silicon steels without si nificant increase incost.

In the copending application of Victor W. Carpenter entitled Method ofproducing silicon steel sheet or strip, 1936, now Patent No. 2,236,519,issued April 1, 1941, there are set forth processes for the productionof silicon steel characterized broadly by a decarburizing treatmentbetween the hot rolling and cold rolling stages, which treatment is abox annealing of the material in the presence of the hot mill scale.This treatment is inexpensive and is effective in reducing the carboncontent to .015% or lower irrespective of the starting carbon incommercial silicon steels. In material of such low carbon content afinal box annealing can be dispensed with, since the grains grow sorapidly that the development of the desired magnetic properties can beachieved in a commercial continuous anneal of the single strand type.The treatment has come into use Ifoor many grades of silicon steelmanufactured Most cold rolled silicon steel is sold on a core lossguarantee. Each commercial grade is made from a selected silicon gradeof bar stock, by a series of operations appropriate to the particularresult desired. In spite of the close adherence to standardizedpractices it has been found that variables in the processing or analysiswhich are not easily controlled may influence the final product to suchan extent that certain batches of material may not meet the core lossguarantee.

' After examination of such batches of material over a long period oftime, we

other factors have been calculated to a comhave found, when parablebasis, that the difiiculty accompanies too small a grain size in thefinished product. We therefore came to the conclusion that greateruniformity in achieving the lower core losses which we obtain, implied'a variation in pro-. cedures to obtain a larger grain size on theaverage;--or, putting it another 'way, greater consistency in corelossresults would follow greater consistency in the production of a large Ithe finished product.

grain size in 7 known that very large grains may 'It has been beobtained in cold rolled silicon steel by strain rolling." This isessentially a finishing operation involving annealing strip when it iswithin a few thousandths of an inch of final gauge, cold rolling it togauge, and

Ser. No. 60,347, filed January-22,

' size, the largerwill be .a working of the piece.

was possible without strain rolling? but which would be available forsubstantially all silicon steel grades because of relatively slightcost. The known operation of strain rolling was therefore not seen asavailable for our purpose. We have determined, however, that the grainsize of the hot-rolled material (or hot-rolled material which has beenannealed) will determine the ultimate grain size in'the cold rolled andannealed product. The larger theoriginalgraln the final grain sizewithin certain limits. Thus, if the original hot-rolled thin bar couldbe made structure, uniformly, we believed we would secure the desiredimprovement and uniformity in the final product. Experience the case. 1There are several methods by which a larger grain structure may besecured in the thin bar.

-A higher annealing temperature following the hotrolling will givelarger grains. Also, if the hot rolling is finished at such atemperature as to leave considerable strain in the thin bar, graingrowth will 'occur during the subsequent box anneal. Conversely, it ispossible to finish the hot be discussed. We find .that good results canbe.

more certainly attained and controlled by introducing strain into thehot-rolled bar afterthe hot rolling so that large grains will growduring the box anneal. There are various methods of accomplishing thispurpose; but they all involve The one or more passes through a scalebreaker, and this will be found to introduce sufiicient strain toproduce useful grain growth or the piece may be strained after the hotrolling by passing it through an ordinary cold mill. Both of theseprocedures can be carried on so as not to remove the cold rolled sheetor I the hot mill scale from the hot-rolled part whereby the eflicacy ofthe following box anneal as a decarburizing treatment is not impaired.The

it. Strain rolling is exto have a larger grain has shown that this ispiece may be given I ing on the first pass, would be strained on the.

second pass. The scale breaker treatment is somewhat less expensive thanthe cold rolling;

. but the use, of the mill for straining results in more uniform grainsize.

We have. examined microscopically materials which have been strained bybeing passed through a scale breaker after the box anneal whichfollows.These materials were found to have large grains on each surface, whichextended toward the midthickness of the bar about one-fourth of the waythrough the bar. Thus the box annealed bars made in this way containedlarge grains which comprised approximately one-half of the totalthickness of the bar. The grains at the midthickness were of the sizewhich would be obtained in. the box annealed bars without the straining.Nevertheless, the amount of material characterized .by large grains inthe box annealed product was found sumcient to produce the desired largegrains after the complete process of making the silicon steel sheets hadbeen gone through. When the pieces strained by cold rolling areexamined, the grain growth due to straining is found to be much moregeneral and uniform throughout the thickness of the piece. a

As an outline of a general process which we heretofore carried'out, -wehot-roll silicon steel to a gauge say from,.06 to .10 inch. We then boxanneal the strip in the presence of the mill scale whereby todecarburize it. Thereafter we pickle the strip, cold roll it to thefinal gauge, and open anneal it. This process as modified in accordancewith the present invention may be summarized as follows: Hot roll tofrom .06 to .10 inch, introduce a critical amount of strain into thebar, box anneal, pickle, cold roll to final gauge, and open anneal. Itwill be noted that we have not added significantly to the cost of theprocess; but the uniformity of results and general improvement inresults which we secure is marked and valuable.

As to the various factorsaffecting grain size of the hot-rolledmaterial, all of these have a certain importance .and combinations ofthem are contemplated by our invention. Even where clnef reliance isplaced upon a definite'working of the metal after it has cooled from thehot rolling temperature, the other factors of conv proper amount ofworking and a proper annealing temperature in view of the other factorsfor the desired grain growth. For example, using a medium silicon ofaround 2% we have found that around 7 to 8% reduction will result in arolled gauges of silicon steel that are now being used. The essentiallimitation upon the hotrolled gauge is that of the power and capacity ofthe hot mills. Moreover, our invention applies to hot-rolled thin barsor the like of any silicon content, which are usable to produce coldrolled strip. The silicon content of the material also is a factor whichhas some bearing upon the amount of strain necessary to produce largegrains.

The application ofour new process results in an enlargement in grainsize over the same process with the new step omitted. It is not to beunderstood that it will produce grains quite as large as those resultingfrom the use of the more expensive strain rolling" process which hasbeen described. The improvement resulting from the use of our process,however, is of such an order as to significantly enhance the magneticproperties of the material.

- Modifications may be made in our invention without departing from thespirit of it. Having thus described our invention, what we claim as newand desire to secure by.Letters Patent is:

1.'A process of producing silicon steel sheet or strip characterized byenlarged grain size, which comprises hot-rolling silicon steel to anintermediate gauge, introducing a critical strain by cold work into thehot-rolled material, box

' annealing it, thereafter cold rolling it to final gauge with a largepercentage reduction substantially in excess of percentage reductionsfor critically straining it, and finally heat treating the siliconsteel, to develop its magnetic properties and a large grain size.

2. A process as claimed in claim 1 wherein the introduction of I thestrain is accomplished by passing the hot-rolled material througha scalebreaken,

3.-The process as claimed in claim 1. wherein the strain is introducedinto the hot-rolled material .by giving it a pass through a cold millprior to the initial annealing.

'4. The process as-claimed in claim 1 wherein,

the strain is introduced into the hot-rolled material by giving ita passthrough a cold mill prior to the initial annealing, the hot-rolledmaterial as passed through said cold mill bearing the hot mill scale onits surfaces.

5. A process of producing low carbon silicon steel or strip whichcomprises hot-rolling silicon steel to an intermediate gauge, coolingthe silicon steel, introducing a critical strain by cold work into thesilicon steel, and thereafter boxannealing the silicon steel withoutremoval of the hot mill scale, thereafter cold rolling the silicon steelto final sheet gauge by producing therein a cold rolling reductionsubstantially in excess of percentage reductions for criticallystraining it, and heat treating it to develop its magnetic propertiesand a large grain size.

6. A process of producing low carbon silicon steel sheet or strip whichcomprises producing hot-rolled silicon steel of an intermediate gaugecharacterized by critical strain productive of a large grain size uponannealing, annealing said silicon steel in the presence of the scaleresulting from hot-rolling, whereby to decarburize it, picklingthe'silicon steel, cold rolling it to final sheet gauge by producingtherein a cold rolling reduction substantially'in excess of percentagereductions for critically straining it, and finally an-.-

nealing the silicon steel to develop its magnetic properties and a largegrain size.

7. A process of producing silicon steel sheet or strip which compriseshot-rolling silicon steel to a gauge of substantially between .06 and.10 inches, introducing critical strain into the hot-rolled siliconsteel, box annealing it, pickling it, cold rolling it to sheet gaugewith percentages of cold rolling reduction substantially in excess ofpercentages for critically straining it, and thereafter open annealingit to develop its magnetic properties and a large grain size.

8. The process as claimed in claim 7 in which the box anneal is carriedon in the presence of the hot mill scale so as to decarburize thesilicon steel as well as cause grain growth therein.

9. A process of producing low carbon silicon steel sheet or strip ofenlarged grain size, which comprises producing hot rolled silicon steel0!.

an intermediate gauge of approximately from .06

to .10 inch, cooling the hot rolled silicon'steel to cold rollingtemperatures, cold rolling it without removing the hot mill scale fromits surfaces, and with only sufllcient elongation to produce a criticalstrain in the silicon steel, thereafter box annealing the silicon steelstill in the presence of the hot mi'llscale, whereby to decarburize itby.

reducing the carbon therein froma higher value to approximately amaximum of .015%, pickling 'the silicon steel, cold rolling it to finalsheet ;vrc'roa' w. CARPENTER.

JOHN M. JACKSON. .mcx E. LUCAS.

